Integrated circuits are fabricated on the surface of a semiconductor wafer in layers and later singulated into individual dies. FIG. 1 shows a cross-sectional view of a semiconductor die assembly 100 including a flip-chip die 110. Die 110, which could be a microprocessor, mechanically and electrically couples to a package substrate 120 through an arrangement of solder bumps 152 on the active face 112 of the die. To attach the die 110 it is first “bumped,” or patterned with solder bumps 152. After bumping, the die 110 may be attached to an upper surface 134 of the package substrate 120 by melting the solder bumps 152, such as in an oven reflow process. An epoxy underfill material 130 may be introduced around the solder bumps 152 to improve solder joint reliability. Electrical communication through the package substrate 120 may be by way of conductive vertical columns, or substrate vias 124. Solder balls 180 attached to the bottom surface 136 of the package substrate 120 may thus allow electrical communication between the die 110 and a printed circuit board (PCB) 190 to which the package substrate 120 may be mounted.
Solder interconnects (e.g., solder bumps 152 between the die 110 and the package substrate 120, or solder balls 180 between the package substrate 120 and the PCB 190) may use similar solder compositions, such as eutectic tin-lead composition. FIG. 2A shows a cross-sectional view of a solder joint 200 prior to a reflow process. The solder joint comprises a solder ball or bump, which will be generically labeled a solder interconnect 210, joined to a solder pad 220. Solder pad 220 has a nickel-gold construction, comprising an outer gold (Au) layer 222, an underlying nickel (Ni) layer 224, and a copper (Cu) base 226. Referring now to FIG. 2B, during the reflow process the solder interconnect 210 is melted and the gold layer 222 is dissolved into the molten solder, exposing underlying nickel layer 224. The nickel layer 224 reacts with solder to form a Ni3Sn4 compound between solder and the nickel layer. After oven reflow, the solder interconnect 210 solidifies.
After reflow, solder joints may be exposed to temperature extremes during use, testing, and possibly storage. Post reflow exposure to elevated temperature, as well as exposure to temperature cycling, may cause the gold, that has been dissolved into a solder interconnect, to migrate toward the nickel layer 224. FIG. 2C shows the solder joint 200 of FIG. 2B after high-temperature exposure and/or exposure to temperature cycles. Above temperatures of about 25° C., gold has an affinity for nickel, and therefore the gold dissolved in the solder may migrate toward the nickel layer 224 in a solid-state diffusion process. However, since tin is also present, the gold tends to react with the tin in the solder to form a composition 228 called “gold-tin-four” (AuSn4) at the interface 230 between the solder interconnect 210 and the exposed nickel layer 224. The gold-tin-four compound 228 is very brittle and greatly weakens the connection between the solder interconnect 210 and solder pad 220. While prior attempts to reduce the creation of AuSn4 reduce the thickness of gold layer 222, this may not be practical with decreasing size of electrical devices.
As technology has advanced, the pitch (or center-to-center distance) P1, P2 between adjacent solder bumps 152 and/or solder balls 180 has decreased. As the pitch decreases, the size of solder pads 114, 184 on which solder bumps 152 and solder balls 180, respectively, rest is reduced, and therefore the size of solder joints 152 and 180 decreases. Gold layer thickness should be set such that after reflow the overall gold content in the solder joint is smaller than the saturation solubility of gold in the solid solder. Therefore, for a smaller solder joint, the gold layer should be thinner. However, the plating thickness of gold required in the solder pads 114, 184 may be thinner than existing plating processes can achieve. Consequently, methods and systems are desired for reducing the embrittlement of solder joints formed between eutectic solder interconnects and nickel-gold plating pads used in electronic or semiconductor devices.